The two hardest materials known are diamond and cubic boron nitride. Because of this, there is considerable research involving both of these materials for applications such as wear-resistant coatings, abrasive coatings and acoustic elements, as well as electronic devices . Currently, however, efforts to grow smooth, homogeneous diamond films on suitable substrates have been unsuccessful, thereby precluding the use of diamond thin films as wear-resistant coatings or in electronic devices. Accordingly, there is a strong need for a method for forming diamond films.
Alternatively, a cubic form of boron nitride has been grown on silicon wafers by means of a laser ablation technique, as disclosed in U.S. patent application Ser. No. 07/446,758 to Gary L. Doll et al, entitled "Laser Deposition of Crystalline Boron Nitride Films", filed on Dec. 6, 1989, and assigned to the same assignee of this patent application. With this laser ablation method, single crystal cubic boron nitride films were epitaxially grown on a silicon substrate oriented along the [100] axis, such that the resulting cubic boron nitride films were in epitaxial registry with the underlying silicon substrate. Two epitaxial registries have been observed for cubic boron nitride on silicon. One epitaxy has the principle axis of a cubic boron nitride with a 0.362 nanometer lattice constant parallel to the crystallographic axes of the silicon, such that three cubic boron nitride lattices oerlay two silicon lattices. The other epitaxy has the [100] direction of a cubic boron nitride with a lattice constant of 0.384 nanometers notched to align with the [110] silicon axis. In this way, two cubic boron nitride lattices overlay one silicon lattice. Since fewer uncompensated silicon bonds exist in the second epitaxy, it is more energetically favorable than the first.
The cubic boron nitride (BN) material is a most interesting III-V compound from both the practical and scientific viewpoints. The boron nitride phase having this cubic crystal structure is particularly useful since it is characterized by many desirable physical properties besides extreme hardness, including high electrical resistivity and high thermal conductivity. In addition, the cubic boron nitride is relatively inert chemically. Because of these properties, this cubic form of the boron nitride is potentially very useful for many applications, including electronic devices, particularly for use at high temperatures.
Therefore, it is clear that the cubic boron nitride has many useful characteristics. However, in order to successfully grow the cubic boron nitride films on the silicon substrate with energetically favorable epitaxy, the crystallographic lattice for the cubic boron nitride must expand to match the lattice constant of the underlying silicon. In particular, the cubic boron nitride films formed on the silicon substrate by the laser ablation method described above, are characterized by a crystallographic lattice constant of approximately 0.384 nanometers as compared to the lattice constant of approximately 0.362 nanometers for bulk cubic boron nitride powder. Thus, the lattice constant for the cubic boron nitride films formed by the laser ablation method is approximately 5 percent larger than the bulk material. Because of this lattice expansion, two cubic boron nitride unit cells can fit along the [110] silicon direction, so as to result in epitaxial registry between the silicon and cubic boron nitride.
Although this lattice expansion brings the cubic boron nitride into crystallographic registry with the underlying single crystal silicon lattice, a large dislocation energy is always associated with a lattice expansion of this magnitude. It is believed that this large dislocation energy may be accommodated by the presence of pinholes and internal stress within the film. Without these mechanisms, or some other vehicle for accommodating this large dislocation energy, the cubic phase for boron nitride is energetically unfavorable.
However, these mechanisms present serious engineering problems, particularly within electronic devices formed form these films. One problem is that there is a substantial electrical current leakage through the pinholes. This effect, as well as other shortcomings, if not corrected makes the cubic boron nitride a less desirable insulator for a silicon based electronic device as compared to other materials. In addition, the cubic boron nitride films which are formed on silicon using this laser ablation technique are not as hard as the bulk cubic boron nitride.
Therefore, it would be desirable to alleviate these shortcomings of the prior art and provide a film which simulates the desired physical and electrical characteristics of both diamond and cubic boron nitride. In particular, it would be desirable to provide such a film using the laser ablation techniques described above.